The invention relates to a method for detecting coking in the intake tract of an internal combustion engine having variable intake camshaft control, and to a corresponding engine testing device for detecting coking.
Coking of the intake tract can occur in gasoline engines having direct fuel injection, in particular on the intake valve (for example, on the valve neck) or in the intake port on the valve opening. With such coking, carbon deposits in the intake tract. The cause for the tendency of gasoline engines having direct fuel injection toward coking is that no continuous cleaning scavenging of the intake tract, for example of the intake valve, with gasoline takes place in engines having direct fuel injection, since the fuel is injected directly into the combustion chamber; oil residue from the crankcase breather can deposit, together with soot particles from the cylinder, in the intake in the form of carbon under the action of heat. The coking tendency increases when low-quality fuels are used and with low-load driving profiles.
This coking causes rough idling (uneven running) when the engine is cold and can result in increased fuel consumption as well as problems in the warm-up driving behavior. A carbon deposit coating at the top on the valve neck between the valve head and the valve stem, for example, can interfere with the tumble flow, so that the idle speed fluctuates to a higher degree and, in the extreme case, even misfires can occur. A carbon deposit coating can even result in performance losses when it acts as a flow resistance due to insufficient cylinder filling. Moreover, a carbon deposit coating on the intake valve or on the valve opening can prevent the valve from closing properly, resulting in compression losses and sporadic misfires. Coking of the intake tract, in particular of the intake valves, in internal combustion engines having direct fuel injection and the consequences thereof are described in the published prior art EP 1 884 637 A2.
The degree of coking in the intake tract is typically determined visually in the workshop only after engine components have been disassembled. If the intact tract exhibits coking, the intake tract is then cleaned or the components in question are replaced.
The visual detection of coking has the drawback of being very labor-intensive due to the disassembly of engine components for visual accessibility. Moreover, the visual inspection is associated with the drawback of a lack of objectivity, since coking is difficult to assess with a purely visual observation.
It is further known from the published prior art DE 199 58 177 A1 that the combustion chamber of an internal combustion engine can also be affected by coking. In connection with the coking of the combustion chamber, it is proposed to detect speed fluctuations of the internal combustion engine and, if the fluctuations in the idle speed are significant, to initiate cleaning of the combustion chambers of the engine.
It is the object of the invention to provide a method for detecting coking in the intake tract of an internal combustion engine having direct fuel injection, which does not require the internal combustion engine to be disassembled for detection and allows an objective assessment of the coking level. The object is further directed to providing an engine testing device having the appropriate properties.
This and other objects are achieved by the present invention.
A first aspect of the invention relates to the detection of coking in the intake tract of an internal combustion engine having direct fuel injection, in particular of a gasoline engine having direct fuel (gasoline) injection. For example, the method is carried out on a vehicle-external engine testing device, which is used in a vehicle repair shop, for example. The engine testing device is connected wirelessly or by wire to the engine control unit of the vehicle via an appropriate vehicle interface and, in this way, is able to control the operation of the vehicle and retrieve measurement values from the engine control unit. However, it is also contemplated for the method to be carried out on the engine control unit of the vehicle, wherein cleaning of the ports is ordered automatically, for example, when coking is detected. For this purpose, a cleaning notice may be issued to the driver or to a service technician in the workshop and/or an appropriate notice indicating coking may be stored in a fault memory of the vehicle. It would also be conceivable for automatic cleaning of the vehicle to be carried out when the engine control unit detects coking, for example by introducing cleaning liquid into the intake tract, or by switching the internal combustion engine to a cleaning operating mode.
The internal combustion engine further has variable intake valve control. This variable intake valve control system allows the opening time of the intake valves in relation to the crank angle of the crankshaft to be modified. For example, this is a variable camshaft control system, in which the position of the intake camshaft relative to the crankshaft is varied, in particular by way of what is known as a vane cell adjuster. It can also be a fully variable valve gear, in which the valve lift can be varied, in addition to the opening time of the intake valves.
In the method, the internal combustion engine is operated in idle at an idle speed that is increased compared to normal idle operation. It may be provided for this purpose that the internal combustion engine is initially operated in normal idle operating mode, for example at a rotational speed in the range of 700 to 900 rpm. For detecting coking, the idle speed is then increased, for example to an idle speed greater than 1200 rpm, in particular to a rotational speed of approximately 1500 rpm.
For conducting the measurement, additionally the opening timing of the intake valves is controlled to achieve an early timing event, whereby the valve overlap (the duration during which the intake valve is already open before the exhaust valve is closed) is increased. It is possible here to institute a particular setting for the opening time and then record the measurement values at this setting of the opening time of the intake valves. For example, a setting for the opening time is instituted, which is in the early half of the control range. It is possible to use a setting at which the intake valves already close just after bottom dead center or already before bottom dead center. However, it is also possible for the opening time to pass once, or multiple times, between advanced and retarded, and for measurement values to be recorded during each passage of the opening time. For example, measurement values in an evaluation window are used for determining a rough running characteristic, the center of gravity of which is, or which is entirely, in the early half of the maximum control range.
By way of the measurement, it is possible to determine a rough running characteristic of the internal combustion engine which is typical of the rough operation of the internal combustion engine. The presence of coking in the intake tract can then be detected based on the rough running characteristic.
The control of the opening time of the intake valves according to the invention to achieve an early timing event essentially increases rough running of the internal combustion engine. The difference in rough running of the internal combustion engine between the case where coking is present in the intake tract and the case where no coking is present thus becomes greater, whereby a distinction between coking and non-coking becomes easier, and coking can thereby be detected more reliably, and lower levels of intake valve coking can also be detected sooner. On the other hand, a higher idle speed should be used with an early opening time of the intake valves so as to prevent the internal combustion engine from stalling.
The method according to the invention allows coking to be objectively determined by measurement, instead of by subjective visual assessment. Cleaning or repairing only takes place when this is objectively truly necessary.
Disassembly of the engine solely for the visual coking assessment is not necessary when using the method. Coking can be determined very quickly by way of the method (for example, within 3 minutes instead of 1 hour or more when disassembling of the engine is necessary).
The rough running characteristic of the internal combustion engine can be compared to a comparison value. If the rough running characteristic increases as rough running increases, for example, it is possible, in particular when the rough running characteristic is greater than the comparison value, to determine that there is a high level of coking of the intake tract and/or a need for cleaning of the intake tract. However, if the rough running characteristic is smaller than the characteristic, there is no need to clean the intake tract.
If the rough running characteristic decreases as rough running increases, for example, it is possible, for example when the rough running characteristic is smaller than the comparison value, to determine that there is a high level of coking of the intake tract and/or a need for cleaning of the intake tract.
The rough running characteristic, the presence of a high level of coking, and/or a need for cleaning can be communicated, in particular visually, to the user of the engine testing device, for example, who will then carry out the cleaning of the intake tract.
Preferably, a cylinder-based rough running characteristic that is typical of the rough operation of a cylinder is determined for each cylinder. The (global) rough running characteristic for the entire internal combustion engine is then formed from the cylinder-based rough running characteristics. This can be done in particular by averaging of the rough running characteristics of the individual cylinders. By averaging the rough running characteristics of the individual cylinders, it is possible, based on the resulting rough running characteristics, to better infer the presence of coking in the intake tract. If the cylinder-based rough running characteristic of all cylinders indicates increased rough running, coking of the intake tract is typically present. If the rough running characteristic of an individual cylinder indicates increased rough running, but the rough running characteristics of the remaining cylinders do not indicate increased rough running, in most cases no coking of the intake tract is present; however, in this case, when averaging the rough running characteristics, the resulting rough running characteristic will precisely not indicate particularly high rough running, since the increase in the rough running characteristic of one cylinder influences the rough running characteristic of the internal combustion engine only by the factor 1/number of cylinders.
The above-described rough running characteristic of the internal combustion engine, in turn, can be an averaged rough running characteristic of the internal combustion engine, which results from averaging multiple rough running characteristics of the internal combustion engine during multiple measurement passes.
The cylinder-based rough running characteristic of each cylinder is preferably determined as a function of multiple rough running values that are based on the particular cylinder, which describe the rough running of the internal combustion engine during the phases of the particular cylinder (which is to say, during firing of the particular cylinder). A rough running value indicates, for example, whether the crankshaft is rotating more quickly or more slowly compared to a reference.
The rough running characteristic of a cylinder can be determined by averaging rough running values that are based on this cylinder. The rough running characteristic of a cylinder can be determined, for example, from the average value of the rough running values that indicate that the crankshaft is rotating more quickly, and from the average value of the rough running values that indicate that the crankshaft is rotating more slowly, for example by determining the difference between these two average values.
Preferably, the engine temperature (based on the temperature of the coolant, for example) is lower or lower than/equal to a temperature threshold value (for example, a temperature threshold value in the range of 30° C. to 50° C., in particular a temperature threshold value of 40° C.), at least during the start of the measurement, so that the measurement is meaningful. This is checked before the measurement, for example. If the engine temperature does not fulfill this condition prior to the measurement, preferably a rapid cool-down of the engine by way of the electric fan is carried out, wherein, for example, the measured temperature of the coolant can be cooled within 2 hours (instead of 8 hours) from 100° C., for example, to 30° C. It could also be provided that the engine temperature must be lower than or lower than/equal to a temperature threshold value during the entire measurement.
A second aspect of the invention is directed to an engine testing device. The engine testing device is configured as a test unit to detect coking in the intake tract of an internal combustion engine having direct fuel injection and variable intake valve control. The device can be connected to a motor vehicle for controlling the operation of the vehicle and for receiving vehicle-internal measurement values, for example via an electrical connection or a radio connection.
The engine testing device includes the test unit and interfaces configured to control the internal combustion engine via the engine control unit in such a way that the engine is operated in idle at an idle speed that is increased compared to normal idle operation. Via the engine control unit, the engine testing device controls the opening time of the intake valves to achieve an early timing event. The engine testing device further determines a rough running characteristic of the internal combustion engine which is typical of the rough operation of the internal combustion engine, wherein the presence of coking in the intake tract is detectable based on the rough running characteristic.
The above comments on the method according to the invention based on the first aspect of the invention apply accordingly to the engine testing device according to the invention based on the second aspect of the invention; advantageous embodiments of the engine testing device according to the invention correspond to the described advantageous embodiments of the method according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.